1. Field of the Invention
The present invention relates to a process for producing iron type magnetic particles which are usually used for a magnetic recording medium.
2. Description of the Prior Arts
It is well known that the important characteristics of magnetic particles for magnetic recording include a coercive force of magnetic particles, a dispersibility of magnetic particles in a magnetic paint for coating and an orientation of magnetic particles in a magnetic recording tape. As it is well known, these characteristics are important factors which highly affect to the characteristics of the magnetic recording tape. The shape of the magnetic particle directly relates to these characteristics of the magnetic particles. For example, when the coercive force is given by the shape anisotropy as the conventional magnetic recording medium, the coercive force is increased depending upon the improvement of the acicular property of the particles. The dispersibility is improved by decreasing branched or curved particles whereby the steric hindrance is decreased to improve the orientation and accordingly, the recording characteristics of the magnetic recording tape are improved. Therefore, it is important to obtain a magnetic particle having excellent acicular property with smaller branched shape in the preparation of the magnetic particles for a magnetic recording medium. In the industrial process, hydrated ferric oxide which is mainly goethite .alpha.-FeOOH and easily forms an acicular particle is firstly produced as a starting material and is converted into a magnetic metallic iron particle by heating, dehydrating and reducing it or is converted into Fe.sub.3 O.sub.4 particles by partially reducing it or is converted into .gamma.-Fe.sub.2 O.sub.3 particles by oxidizing the product. These process have been usually employed.
In this process, the acicular property of the resulting particles depends on the shape of the starting goethite and accordingly, the shape of the goethite particles should be controlled. But in the conventional type of the particles, the deformation and sintering of the particles are disadvantageously easy to occur by the dehydration and the migration of atoms in the reduction process. This causes the limitation of the orientation in the tape as described below.
On the other hand, it is found that the growth of crystals of the particles is remarkably inhibited by Si component. There are many proposals based on the phenomenon as disclosed in Japanese Examined Patent Publication No. 19541/1977; and Japanese Unexamined Patent Publication No. 95097/1977; No. 134858/1977; No. 4895/1978; No. 30758/1977; No. 77900/1976; No. 121799/1977; and No. 153198/1977. Various processes for incorporating Si component in the acicular non-magnetic particles as the starting material can be considered as proposed. In the fundamental consideration, these processes can be classified into a process for growing crystals in the presence of Si component in the step of the production of the goethite as the starting material (referring to as A process) and a process for adsorbing Si component as a post treatment, after producing a non-magnetic particles (referring to as B process).
The A process is superior to the B process in view of the uniform incorporation of the Si component, however, it has disadvantage that the growth of the goethite crystals is inhibited to be difficult to obtain the particles having the desired size if the Si content is increased. The B process has not any difficulty on the control of the particle size, however, it is difficult to adsorb uniformly the Si component and the bonding of the Si component to the surface of the particles is not so strong to effectively impart the effect of the Si component. On the contrary, the present invention is to combine both the advantages to overcome the disadvantages.